Normally the auxiliary verb to do is used in negative and interrogative sentences; the emphasizing do is used in Affirmative and Imperative ones.

You did know about her illness and did nothing to help her.
Вы же знали о ее болезни и ничего не сделали, чтобы помочь ей.

But I do love her!
Но я же люблю ее!

Do come and see us on Sunday.
Непременно приходите к нам в воскресенье.